Not applicable.
The present invention relates to a fastener for holding two objects in a fixed spatial relationship with respect to one another, and more particularly to a screw-type fastener, and associated driving device, for fixating two bone segments in a fixed relationship.
The use of screw fixation to hold bone segments has been established as common practice in the surgical management and treatment of bone fractures. When fastening two segments together, it is often desirable to use a lag screw, having threads engaged in a distal bone segment and a smooth shank engaged in a proximal bone segment. Rotably engaging the fastener develops tension in the fastener assembly by advancing the distal bone segment along the long axis of the fastener until contact is made with the proximal bone segment. As the fastener assembly is further tightened, compressive stress is developed at the mating interface of the two bone segments, which has been shown to assist the fracture healing process.
As is well known, any device implanted into the human body, however, causes some type of tissue reaction to the implanted foreign material. For this reason, it is desirable to use the smallest feasible fasteners for fixating bone segments. Also, in some instances, there are definite anatomic limits to the size of fastener which may be employed for a particular condition.
One area where the anatomic limit is particularly notable is in anterior odontoid fracture fixation. In this condition, the dens portion of the second cervical vertebra has fractured, resulting in a bone segment being loose within the ring formed by the first cervical vertebra. Normally, the dens acts as a pivot around which the first cervical vertebra rotates. However, this pivot function is lost when the dens is fractured from the body of the second cervical vertebra. In order to restore the ability of the first cervical vertebra to rotate, and to prevent injury to the spinal cord, the dens is surgically accessed by an anterior inferior approach. An angled hole is drilled through the third cervical vertebra starting from the anterior face of the vertebral body, and leading in a path stopping at the inferior endplate of the second cervical vertebra. A smaller screw hole is drilled and then tapped from the caudal endplate of the second cervical vertebra through the center of the fractured dens. A screw, preferably a lag screw, is inserted through the body of the second cervical vertebra and into the fractured dens using some form of driving device. Rotably engaging the screw draws the fractured bone segment to the cephalad surface of the second cervical vertebra.
Effectively, the screw acts as a mechanical support for the dens to restore its function of acting as a pivot for the first cervical vertebra. The screw also allows compressive force to be developed at the fracture faces of the dens and the body of the second cervical vertebra, assisting the fracture healing process.
Surgical experience shows that standard straight-shank screws of the maximal diameter anatomically allowed often are insufficient for the surgical management of odontoid fractures. Often, the implant itself will fracture during healing due to high bending stresses imparted by the first cervical vertebra on the dens, and thereby in turn on the implant. Failures usually occur in areas of high stress concentrations, such as are developed in straight-shank beams loaded in bending conditions. Surgical experience also shows that, during the treatment of bone fractures and other surgical procedures, implantable fasteners can slip from the driving devices and into the patient.
Accordingly, an improved fastener is required to surgically manage odontoid fracture, effectively fixate other structures, and to prevent the accidental entry into the human body.
The bone fastener of the present invention addresses and overcomes problems found in the prior art. In one aspect of the invention, a fastener for fixating two bone segments is provided, wherein the fastener has a helical thread portion for engaging a distal bone segment, a shank portion for spanning a proximal bone segment, and a head portion for acting as a stop or brake against the proximal bone segment. Also, a means to drive the fastener into the bone segments is provided.
In another aspect of the present invention, the thread portion of the fastener may be any sort of helical screw thread. It may be right-handed, left-handed, a machine thread, a cancellous bone thread, a buttress thread, or any other thread as is known in the art. Though a threaded fastener is described in a preferred embodiment, it is contemplated that the threaded portion may be replaced or augmented by non-threaded fastener means, such as bone hooks or anchors, or expanding barbs, including means which may be developed through the use of shape memory alloys.
In yet another aspect of the present invention, the shank portion of the present invention has a circular cross-section with a diameter that varies along the length of the shaft in a manner such that the resultant stress developed due to load applied in the distal bone segment is always a constant value. Maintaining the resultant stress at a constant value minimizes the development of stress concentrations and optimizes the strength vs. size relationship of the implanted component.
In a further aspect of the present invention, the head portion of the bone fastener has a diameter larger than the largest diameter of the shank portion of the fastener, so that the head portion acts as a stop against the proximal bone segment. In one present embodiment, the head portion also has a broached hex cavity and a smaller diameter internally threaded hole to facilitate driving and removing the fastener with a unique driving instrument.
In a final aspect, a driving instrument has a ball-type hex driver at its tip for engaging the broached hex cavity located in the head portion of the screw fastener. The driving instrument is further cannulated and accommodates a draw rod for engaging the threaded hole of the head portion of the bone fastener. Rotably engaging the draw rod while the ball-type hex is seated into the broached hex develops tension in the draw rod and bone fastener assembly, thereby firmly drawing the bone fastener to the driving instrument. The bone fastener may be driven without fear of it dropping off of the instrument and into the patient. Once the threaded portion of the fastener is engaged with the first bone segment, the draw rod may be removed, thereby allowing the ball-hex driver to drive the fastener at an angle from its centerline.